1. Field of the Invention
The present invention relates to a spark plug for use on internal-combustion engines and a method for producing it.
2. Description of the Related Art
The spark plug used to ignite internal-combustion engines such as automotive gasoline engines comprises a central electrode, an insulator provided outside the central electrode, a main metal shell provided outside the insulator, and a ground electrode fitted on the main metal shell in a face-to-face relationship to define a spark discharge gap. A mounting threaded section is formed on the peripheral surface of the main metal shell and by means of this thread section the spark plug is mounted on the cylinder head of the engine for subsequent service.
The main metal shell is typically made of ferrous materials such as carbon steels and its surface is in most cases galvanized for protection against corrosion. The zinc plate layer exhibits good anti-corrosive action on iron. However, as is well known, the zinc plate layer on the iron substrate will easily be consumed by sacrificial corrosion, with the additional disadvantage that its appearance is prone to be damaged by the resulting zinc oxide which causes white discoloration. To deal with this problem, most spark plugs have the surface of the zinc plate layer further coated with a chromate coat so that it is protected against corrosion.
Spark plugs have heretofore been coated with the so-called xe2x80x9cgold chromate coatxe2x80x9d on the main metal shell. Because of its high performance in corrosion protection, the gold chromate coat is extensively used in non-spark plug applications such as coatings on the inner surfaces of food reserving cans. However, since part of the chromium component is contained in the hexavalent form, the use of the gold chromate coat is being gradually discouraged by the increasing global concern for environmental protection. In the automotive industry which is a massive user of spark plugs, a total ban in the future on the use of chromate coatings containing hexavalent chromium is being reviewed considering the possible environmental impact of waste spark plugs. As a further problem, the treating baths for depositing the gold chromate coatings contain relatively high concentrations of hexavalent chromium and huge cost is necessary to treat the waste effluents.
Under these circumstances, the development of chromate coatings free from hexavalent chromium, or those in which substantially all chromium component is in the trivalent form, has been underway for comparatively many years. The treating baths proposed to date are mostly low in the concentration of hexavalent chromium and some of them are entirely free from this form of chromium, thus contributing to alleviating the problem of effluent treatment. However, a major defect of the trivalent chromium based chromate coatings is that they are inferior to the gold chromate coatings in the ability to prevent corrosion and this is why they have not found extensive use as coatings to cover the main metal shell of spark plugs.
In addition, the chromate coatings including the gold chromate coat share the common problem of being low in heat resistance. In automotive engines, the cylinder head on which the spark plug is mounted is water cooled and the spark plug seldom becomes extremely hot. However, if the engine is continuously run under high thermal load conditions or if the mounting position of the spark plug is fairly close to the exhaust manifold, the temperature of the main metal shell sometimes increases to about 200 to 300xc2x0 C. In this situation, the chromate coat undergoes accelerated deterioration and its performance in corrosion protection may suddenly drop.
It is an object of the present invention to provide a spark plug having the surface of the main metal shell coated with a chromate coat that is reduced in the content of hexavalent chromium and which yet exhibits better anti-corrosion performance and higher heat resistance than the conventional chromate coatings.
It is another object of the invention is to provide a method for producing the spark plug.
[Means for Attaining the Objects, Mode of its Action and Resulting Advantages]
According to a first aspect of the present invention, a spark plug comprises a central electrode, an insulator provided outside said central electrode, a main metal shell provided outside said insulator and a ground electrode provided to oppose to said central electrode to define a spark discharge gap, wherein the surface of said main metal shell is coated with a complex chromate coat containing a chromium component and a phosphorus component as cationic components, at least 90 wt % of the chromium component being trivalent chromium and the phosphorus component being present in an amount of 1 to 15 wt % as calculated for PO4.
The xe2x80x9ccationic componentsxe2x80x9d as used herein is a term related to a photoelectron spectrum for a coating analyzed by X-ray photoelectron spectroscopy (XPS or ESCA) and means any component of interest (element or atom) that has a chemical shift in the peak of its binding energy toward a positive ionic valence.
In the above spark plug, the surface of the main metal shell is coated with a complex chromate coat that contains a chromium component and a phosphorus component as cationic components, at least 90 wt % of the chromium component being trivalent chromium and the phosphorus component being present in an amount of 1 to 15 wt % as calculated for PO4 . In the ordinary gold chromate coat, about 25 to 35 wt % of the chromium component is hexavalent chromium. However, in the coating of the invention, the content of hexavalent chromium is less than 10 wt % of the chromium component, which is small enough to be of benefit to environmental protection by reducing the emission of hexavalent chromium. The treating bath used to deposit the chromate coating of the invention contains no hexavalent chromium at all or contains only a small amount of it as compared with the treating baths for the gold chromate coat and other conventional chromate coatings. As a result, the problems with effluent treatment are substantially reduced.
The complex chromate coat used in the spark plug is characterized by containing a phosphorus component as a cationic component. The complex chromate coat containing a phosphorus component is markedly improved over the ordinary trivalent chromium based chromate coat in terms of the ability to prevent corrosion, giving the main metal shell of the spark plug adequate durability against corrosion.
If the content of the phosphorus component in the complex chromate coat is less than 1 wt %, the desired performance in corrosion prevention is not attained. Incorporating more than 15 wt % of the phosphorus component is very difficult, since there is a limit on the concentration of the phosphorus component in the treating bath to be used. The content of the phosphorus component in the complex chromate coat is more desirably in the range of 5 to 10 wt %. For the purpose of enhancing the corrosion preventing performance of the complex chromate coat, it is desired that the phosphorus component be mainly contained in the form of phosphate ion (PO43xe2x88x92).
The complex chromate coat may contain a phosphorus component dispersing chromate layer that has the phosphorus component dispersed in a trivalent chromium based compound in an amount of 2 to 15 wt % as calculated for PO4. The phosphorus component dispersing chromate layer can be easily formed by immersing the main metal shell of a spark plug into a chromating bath containing phosphoric acid or a phosphate. The dispersion of the phosphorus component in the trivalent chromium based compound contributes to a further improvement in the corrosion preventing performance of the complex chromate coat. In this case, the phosphorus component originates from the phosphoric acid or phosphate contained in the chromating bath.
The phosphorus component dispersing chromate layer may independently constitute the whole of the complex chromate coat. In this case, from the viewpoint of ensuring the desired performance in corrosion prevention, the concentration of the phosphorus component in the phosphorus component dispersing chromate layer (i.e., the complex chromate layer) is desirably at least 2 wt % as calculated for PO4. In order to improve the corrosion protecting and heat resisting capabilities, the phosphorus component dispersing chromate layer may be coated with another layer such as a silica based or siliceous layer to be described hereinafter. In this case, the concentration of the phosphorus component throughout the complex chromate coat may be as small as about 1 wt %. On the other hand, due to the limit on the concentration of the phosphorus component in the chromating bath to be used, it is very difficult to form a phosphorus component dispersing chromate layer containing more than 15 wt % of the phosphorus component as calculated for PO4.
According to a second aspect of the present invention, the spark plug comprises a central electrode, an insulator provided outside said central electrode, a main metal shell provided outside said insulator and a ground electrode provided to oppose to said central electrode to define a spark discharge gap, wherein the surface of said main metal shell is coated with a complex chromate coat that contains a chromium component and a silicon component as cationic components, at least 90 wt % of the chromium component being trivalent chromium and the silicon component being present in an amount of 5 to 75 wt % as calculated for SiO2.
In this spark plug, also, the content of hexavalent chromium in the complex chromate coat is less than 10 wt % of the chromium component, which is small enough to be of benefit to environmental protection by reducing the emission of hexavalent chromium. In addition, the problems with effluent treatment are substantially reduced. What is more, due to the inclusion of the silicon component as a cationic component, the complex chromate coat is markedly improved over the ordinary trivalent chromium based chromate coat in terms of the ability to prevent corrosion and withstand heat, allowing the main metal shell of the spark plug to have significantly improved durability against corrosion.
If the content of the silicon component in the complex chromate coat is less than 5 wt %, it becomes difficult to ensure the desired ability to prevent corrosion and withstand heat. If the content of the silicon component exceeds 75 wt %, the relative proportion of the chromate compound decreases and the performance in corrosion protection is impaired rather than improved. Note that the content of the silicon component in the complex chromate coat is more desirably in the range of 10 to 40 wt %.
The complex chromate coat may contain a silicon component dispersing chromate layer that has the silicon component dispersed in a trivalent chromium based compound in an amount of 10 to 40 wt % as calculated for SiO2. The silicon component dispersing chromate layer can be easily formed by immersing the main metal shell of a spark plug into a chromating bath containing an alkali silicate. The dispersion of the silicon component in the trivalent chromium based compound contributes to a further improvement in the ability of the complex chromate coat to prevent corrosion and withstand heat. In this case, the silicon component originaterom the alkali silicate contained in the chromating bath.
The silicon component dispersing chromate layer may independently constitute the whole of the complex chromate coat. In this case, from the viewpoint of ensuring the desired ability to prevent corrosion and withstand heat, the concentration of the silicon component in the silicon component dispersing chromate layer (i.e., the complex chromate layer) is desirably at least 10 wt % as calculated for SiO2. In order to further improve the corrosion protecting and heat resisting capabilities, the silicon component dispersing chromate layer may be coated with another layer (e.g. a resin layer or a siliceous based layer to be described later). If the silicon component dispersing chromate layer is to be coated with a silicon-free layer, the concentration of the silicon component throughout the complex chromate coat may be as small as about 5 wt %. Conversely, if the silicon component dispersing layer is to be coated with a silicon-containing layer (e.g. a silica layer), the concentration of the silicon component throughout the complex chromate coat may be as large as 75 wt %. It should be noted that due to the limit on the concentration of the alkali silicate in the chromating bath to be used, it is very difficult to form a silicon component dispersing chromate layer containing more than 40 wt % of the silicon component as calculated for SiO2.
The phosphorus component dispersing layer and the silicon component dispersing layer may be combined into a complex structure (which may be regarded as containing 10 to 40 wt % of the silicon component in the phosphorus component dispersing chromate layer). In this case, the improvement in corrosion preventing performance achieved by the dispersion of the phosphorus component is combined with the improvement in the ability to prevent corrosion and withstand heat as achieved by the dispersion of the silicon component, so that a complex chromate coat of even better performance is produced.
For the purpose of enhancing the corrosion preventing performance of the complex chromate coat, it is desired that the phosphorus component in the phosphorus component dispersing chromate layer be mainly contained in the form of phosphate ion (PO43xe2x88x92). For the purpose of enhancing the corrosion preventing and heat resisting capabilities of the complex chromate coat, it is desired that the silicon component be mainly contained in the form of a silicon compound such as silicon dioxide (SiO2). In the present invention, the phosphorus component is assumed to be bonded to oxygen if a phosphorus peak with a valence of +5 or a value close to it and an oxygen peak with a valence of xe2x88x922 or a value close to it are detected simultaneously in an XPS spectrum. If a silicon peak with a valence of +4 or a value close to it and an oxygen peak with a valence of xe2x88x922 or a value close to it are detected simultaneously, the silicon component is assumed to be bonded to oxygen.
A chromate coat is formed by the reaction between the substrate or base metal and the solution containing chromate ions. This reaction is said to proceed mainly by the following mechanism: trivalent chromium atoms are connected together by bridges of a hydroxyl group and oxygen to form a polymer to like complex which is precipitated and deposited as a gel on the surface of the substrate metal. If a hydroxyl group binds to tetravalent chromium, the proton in the hydroxyl group causes an apparent shift to +4 in the valence of chromium. In the present specification, the chromium component is assumed to be a constituent of the chromate coat if an XPS spectrum has a peak component with a chemical shift from the peak position for trivalent chromium to a position that generally corresponds to a valence of +4.
Chromating is a kind of chemical conversion treatment in which the chromium component is xe2x80x9csubstitutionxe2x80x9d deposited on the substrate metal as the latter is oxidatively dissolved. Therefore, in an electroless chromating method which has no external power supply, the substrate metal must be capable of dissolving into the chromating bath. The main metal shell and the gasket of a spark plug are generally formed of ferrous materials such as carbon steels. In order to protect them from corrosion, their surfaces can be coated with a zinc based plate layer of which the metal component is mostly composed of zinc. The zinc based plate layer can advantageously be used as the substrate for the formation of a chromate coat since it is capable of dissolving into the chromating bath. In this case, the dissolved zinc component is very often incorporated into the chromate coat. The zinc based plate layer can be formed by known electrolytic galvanizing or hot zinc dipping techniques.
If an electrolytic chromating method is adopted, a chromate coat can be formed on a nickel based plate layer of which the metal component is mostly composed of nickel.
In order to ensure the required performance in corrosion protection, the phosphorus component dispersing chromate layer or the silicon component dispersing chromate layer is desirably such that the total weight of the cationic components minus the weight of the phosphorus component or the silicon component is occupied by the chromium component in a total weight of at least 50 wt %. In this case, the cationic components other than the chromium component may be comprised of zinc, nickel, etc.
A siliceous layer based on a silicon oxide may be formed in the surface layer of the complex chromate coat and this helps further enhance its ability to prevent corrosion and withstand heat. As in the case of the phosphorus component dispersing chromate layer and the silicon component dispersing chromate layer, the siliceous layer is desirably such that the total weight of the cationic components is occupied by the silicon component in a total weight of at least 50 wt %, with the balance being composed of other cationic components such as chromium, zinc and nickel.
In order to form the siliceous layer, the main metal shell of a spark plug having a substrate trivalent chromium based chromate layer formed on its surface may be coated with a silicate solution having an alkali silicate dissolved in a suitable solvent which is subsequently evaporated. The resulting siliceous layer is mainly composed of an oxide in which the cationic components are mostly an alkali metal element and silicon. The substrate trivalent chromium based chromate layer is exemplified by the phosphorus component dispersing chromate layer or the silicon component dispersing chromate layer, provided that they may be replaced by a chromate layer that does not contain the phosphorus component or the silicon component in an amount exceeding the lower limits for their inclusion in the phosphorus component dispersing chromate layer or the silicon component dispersing chromate layer, on the condition that the overall complex chromate coat including the siliceous layer contains the silicon component in an amount of 5 to 75 wt %
Alternatively, the siliceous layer may be formed by vapor-phase film deposition techniques such as high frequency sputtering, reactive sputtering, ion plating and chemical vapor deposition (CVD). However, the application of the silicate solution is preferred, since the siliceous layer can be formed by simply immersing the chromated main metal shell (or gasket) in the silicate solution or spraying it with the silicate solution or otherwise applying the solution to form a coat, which is then dried.
Between the substrate trivalent chromium based chromate layer and the siliceous layer, there may be formed a trivalent chromium/silicon dispersing layer which has the trivalent chromium component and the silicon component dispersed in proportions that are smaller than the respective contents in the first two layers. This occasionally contributes to a further improvement in the ability of the complex chromate coat to prevent corrosion or withstand heat. The trivalent chromium/silicon dispersing layer is a kind of compositionally gradient structure that is formed between the trivalent chromium based chromate layer and the siliceous layer and the above-described improvement in the performance of the complex chromate coat can be achieved by several reasons such as betterment of the adhesion between the trivalent chromium based chromate layer and the siliceous layer and relief of the stress due to differential shrinkage of the two layers during heating.
It is generally considered that the gold chromate coat and other existing hexavalent chromium based chromate coatings exhibit satisfactory performance in corrosion protection, since the hexavalent chromium contain therein helps to repair the reticulate structure of trivalent chromium atoms even if the protective coat breaks in a corrosive environment. However, the trivalent chromium based chromate layer does not have this repair effect of the hexavalent chromium. If pin holes and other defects develop in the protective coat, the corrosive action would directly affect the substrate such as the zinc based plate layer to cause rapid progress of the corrosion. However, in the complex chromate coat of the invention having the siliceous layer, the trivalent chromium based chromate layer is xe2x80x9covercoatedxe2x80x9d with the siliceous layer and the corrosive action would have a long way to go before reaching the surface of the underlying trivalent chromium based chromate layer. Hence, the substrate layer contributes to enhance corrosion protection.
The conventional chromate coatings have only poor heat resistance, because they would shrink with heat to increase the likelihood for the occurrence of defects such as cracks. This is not the case with the complex chromate coat of the invention having the siliceous layer; even if cracks and other defects develop in the trivalent chromium based chromate layer, the overlying heat-resistant siliceous layer would retard the deterioration of the corrosion preventing performance of the complex chromate layer.
In order to form a uniform siliceous layer, it is also important to increase the wettability of the substrate trivalent chromium based chromate layer with the silicate solution. For example, when the substrate trivalent chromium based chromate layer has pin holes, cracks or any other defects (which may be the defects in the substrate that were initially caused by surface flaws, the deposition of foreign matter, etc.), if the silicate solution is not capable of efficient wetting of the substrate trivalent chromium based chromate layer, there is high likelihood for bubbles and other unwanted phenomena to be trapped within the defects. This problem can be effectively solved by using an aqueous silicate solution that contains a suitable amount of surfactant.
There is still another way to form the siliceous layer. After the end of the chromating step, the main metal shell of the spark plug may be dipped in the silicate solution while it remains either yet to be dried or partially dried on the surface. That is, the as-chromated wet or half dry surface of the main metal shell has the slightly moistened substrate trivalent chromium based chromate layer which has good affinity for the aqueous silicate solution which is subsequently applied. Even if defects have been formed in the substrate trivalent chromium based chromate layer, the aqueous silicate solution sufficiently fills the defects that they will not trap any undesirable phenomena such as bubbles and the corrosion preventing performance of the complex chromate coat is improved. As another advantage, the chromating solution remaining on the surface of the substrate trivalent chromium based chromate layer mixes with part of the applied aqueous silicate solution to increase the chance of the formation of the already mentioned trivalent chromium/silicon dispersing layer. We have already explained the advantages of forming the trivalent chromium/silicon dispersing layer.
Some spark plugs have an annular gasket fitted around the basal end portion of the mounting threaded section formed on the peripheral surface of the main metal shell. When the threaded section of the main metal shell is screwed into the threaded hole in the cylinder head, the gasket deforms to be compressed between a gas seal flange formed at the distal end of the threaded section and the peripheral edge of the opening of the threaded hole so as to provide a seal between the threaded hole and the gas seal flange. In this type of spark plug, the surface of the gasket may at least partly be coated with the above-described complex chromate coat of the invention so that not only the main metal shell but also the gasket can be provided with the desired corrosion and heat resisting properties.
If desired, a zinc plate layer as the substrate metal layer may be overlaid with the complex chromate coat of the invention. In a spark plug having this layer arrangement, when subjected to xe2x80x9c5. Neutral Salt Spray Testxe2x80x9d according to the plate corrosion resistance test procedure specified in JIS H8502, it can withstand for at least 40 hours before at least about 20% of the whole surface is coated with white rust due to corrosion of the zinc plate layer. This is a satisfactory level of corrosion resistance that should be exhibited by the main metal shell of spark plugs.
The problem peculiar to spark plugs is that if the engine is continuously run under high thermal load conditions or if the mounting position of the spark plug is fairly close to the exhaust manifold, the temperature of the main metal shell sometimes increases to about 200 to 300xc2x0 C. These situations can be effectively dealt with by forming a zinc plate layer as the substrate metal layer and overlying it with the complex chromate coat of the invention and a spark plug having this layer arrangement exhibits satisfactory enduring performance in the following test simulating those situations. Thus, the spark plug is characterized in that when subjected to xe2x80x9c5. Neutral Salt Spray Testxe2x80x9d according to the plate corrosion resistance test procedure specified in JIS H8502 after heating at 200xc2x0 C. for 30 minutes in air atmosphere, it can withstand for at least 40 hours before at least about 20% of the whole surface is coated with white rust due to corrosion of the zinc plate layer.